Short answers, pulled from the story.
The English word guitar comes from the Spanish guitarra, which derives from the Andalusian Arabic qitharah and the Latin cithara, which in turn came from the Ancient Greek kithara. The German Gitarre and French guitare were also adopted from the Spanish guitarra.
Around 1850, Spanish makers including Manuel de Soto y Solares developed the modern guitar's form, but Antonio Torres Jurado is considered the most important. He increased the size of the body, altered its proportions, and invented the fan-braced pattern that greatly improved volume, tone, and projection. His design has remained essentially unchanged since.
George Beauchamp invented the first successful magnetic pickup for a guitar. It was incorporated into the 1931 Ro-Pat-In Frying Pan lap steel, made by the company that later became Rickenbacker. Electric guitars were first patented in 1937.
Standard tuning runs from a low E to a high E across a two-octave range, written EADGBE. It is a series of ascending fourths with a single major third between the second and third strings. By the 16th century the tuning ADGBE had been adopted, and a lower E was later added as a sixth string.
There are three main types of modern guitar: the classical or Spanish guitar, the steel-string acoustic guitar or electric guitar, and the Hawaiian guitar played across the player's lap. Guitars are broadly divided into acoustic and electric categories, each with many subcategories such as flat-top, archtop, resonator, and twelve-string.
All three principal types of resonator guitar were invented by the Slovak-American John Dopyera, who lived from 1893 to 1988, for the National and Dobro companies. Dobro is short for Dopyera Brothers. The sound is produced by one or more aluminum resonator cones mounted in the middle of the top.
The truss rod is a thin, strong metal rod inside the neck that corrects changes to the neck's curvature caused by aging timbers, humidity, or string tension. Turning it clockwise straightens the neck, while turning it counter-clockwise allows a forward bow. Classical guitars do not require truss rods because their nylon strings exert lower tension.